I. Technical Field
The present invention relates to power supplies. More specifically, the present invention relates to a novel uninterruptible power supply (UPS) for supplying continuous output power from at least one of two sources of input power.
II. Background Art
Uninterruptible power supplies are used in applications where it is necessary that continuous power be supplied to a load. One such application of the uninterruptible power supply is in the field of medical equipment wherein if a loss of input AC power occurs, it is necessary that the equipment continue to function under standby power. Another application of the uninterruptible power supply is in computer equipment. In the event of an input AC power failure, standby power would permit the operator to continue computing without loss of data in volatile memories. In computer applications, the standby power must be applied such that the transition from the input AC poewr source to the standby power source does not cause a loss of data.
Previous applications of uniterruptible power supply have employed two techniques. The first technique is the "on-line" power supply wherein the AC input power is constantly applied to a battery through a battery charger. The battery supplies direct current to an inverter which outputs AC power at the same frequency and magnitude as the input AC power. The disadvantages of the "on-line" power supply is that two inverters are required, one to convert the input AC current and voltage to a DC current and voltage to charge the battery and another to convert the DC current voltage to an output AC current and voltage. Typically "on-line" power supplies require transformers which can be rather expensive, heavy, and large in size. "on-line" power supplies tend to be inefficient and generate large amounts of heat that must be dissapated.
Another technique used in uninterruptible power supplies is the "off-line" technique wherein AC power is supplied directly to the load and upon detection of a loss in input AC power, a battery powered inverter supplies output power to the load. Switching between input AC power and power supplied by a battery-powered inverter results in a transisitn time which may result in an instantaneous loss of power to the load. This temporary loss of power can result in the load function failure such as the loss of data stored in volatile computer memories. Typically, power supplies utilizing the "off-line" technique, generate a square wave output rather than the sinusoidal output required in many applications.
It is therefore, an object of the present invention to provide an uninterruptible power supply which eliminates the transition time between switching from one source of power to another source of power so as to supply continuous output power.
It is another object of the present invention to provide highly efficient uninterruptible power supply that is compact in size and weight.